The present invention relates generally to the field of imaging, and more specifically to the field of tomosynthesis. In particular, the invention relates to tomosynthesis systems and methods employing new scanning trajectories for an x-ray source and image acquisition points for a detector to yield an improved image of an object.
Tomography is well known for both industrial and medical applications. Conventional tomography is based on a relative motion of the x-ray source, the detector and the object. Typically, the x-ray source and the detector are either moved synchronously on circles or are simply translated in opposite directions. Due to that correlated motion, the location of the projected images of points within the object moves also. Only points from a particular slice, typically called a focal slice, will be projected always at the same location onto the detector and therefore imaged sharply. Object structures above and below the focal slice will be permanently projected at different locations. Because of that, they aren't imaged sharply and will be superimposed as a background intensity to the focal slice. This principle of creating a 3D image with one slice in focus (focal slice) using a discrete number of projections is called tomosynthesis.
Tomosynthesis systems for medical applications, typically use an x-ray source for producing a fan or cone-shaped x-ray beam that is collimated and passes through the patient to then be detected by a set of detector elements. The detector elements produce a signal based on the attenuation of the x-ray beams. The signals may be processed to produce a radiographic projection. The source, the patient, or the detector are then moved relative to one another for the next exposure, typically by moving the x-ray source, so that each projection is acquired at a different angle.
By using reconstruction techniques, such as filtered backprojection, the set of acquired projections may then be reconstructed to produce diagnostically useful three-dimensional images. Because the three-dimensional information is obtained digitally during tomosynthesis, the image can be reconstructed in whatever viewing plane the operator selects. Typically, a set of slices representative of some volume of interest of the imaged object is reconstructed, where each slice is a reconstructed image representative of structures in a plane that is parallel to the detector plane, and each slice corresponds to a different distance of the plane from the detector plane.
In addition, because tomosynthesis reconstructs three-dimensional data from projections, it provides a fast and cost-effective technique for removing superimposed anatomic structures and for enhancing contrast in in-focus planes as compared to the use of a single x-ray radiograph. Further, because the tomosynthesis data consists of relatively few projection radiographs that are acquired quickly, often in a single sweep of the x-ray source over the patient, the total x-ray dose received by the patient is comparable to the dose of a single conventional x-ray exposure and is typically significantly less than the dose received from a computed tomography (CT) examination. In addition, the resolution of the detector employed in tomosynthesis is typically greater than the resolution of detectors used in CT examinations. These qualities make tomosynthesis useful for such radiological tasks as detecting pulmonary nodules or other difficult to image pathologies.
Though tomosynthesis provides these considerable benefits, the techniques associated with tomosynthesis also have disadvantages.
Reconstructed data sets in tomosynthesis often exhibit a blurring of structures in the direction of the projections that were used to acquire the tomosynthesis data. This is expressed in a poor depth resolution of the 3D reconstruction or depth blurring. These artifacts associated with an imaged structure will vary depending on, the orientation of the structure with respect to the acquisition geometry. For example, a linear structure which is aligned with the linear motion of a linear x-ray tomosynthesis system, will appear blurred throughout the depth of the volume of interest, whereas such a structure will be blurred much less by the circular motion of a circular x-ray tomosynthesis system. The blurring of structures may create undesirable image artifacts and inhibit the separation of structures located at different heights in the reconstruction of the imaged volume.
Therefore there exists a need to adapt the current tomosynthesis systems to provide for new scanning trajectories and image acquisition points to address the depth blurring of the imaged object.